Add LZ4 compression support for TCP transfers

- Add bundled LZ4 library (lz4.h, lz4.c)
- Add conn_options_t for connection-level mode + compression negotiation
- Add COMPRESS_NONE and COMPRESS_LZ4 constants
- Extend file_meta_t with compress and compressed_size fields
- Client: compress files before sending when compression enabled
- Server: decompress received files when compression flag is set
- Add -c flag to DISABLE compression (default: ON)
- Update Makefile to compile lz4.o
- Compression falls back to uncompressed if LZ4 doesn't reduce size

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

Makefile

@@ -5,15 +5,18 @@ TARGETS = fastsync_server fastsync_client
 
 all: $(TARGETS)
 
-fastsync_server: src/server.o src/utils.o
+fastsync_server: src/server.o src/utils.o src/lz4.o
 	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
 
-fastsync_client: src/client.o src/utils.o
+fastsync_client: src/client.o src/utils.o src/lz4.o
 	$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
 
 src/%.o: src/%.c src/common.h
 	$(CC) $(CFLAGS) -c -o $@ $<
 
+src/lz4.o: src/lz4.c src/lz4.h
+	$(CC) $(CFLAGS) -c -o $@ $<
+
 clean:
 	rm -f src/*.o $(TARGETS)